1. Field of the Invention
The present invention relates to a direct color thermal printing method, and more particularly to a direct color thermal printing method wherein black characters are prevented from being blurred. The present invention further relates to a direct color thermal printer for this method.
2. Related Art
A thermosensitive color recording medium has been suggested, for example, in Japanese Laid-open Patent Application 61-213169, and U.S. patent application Ser. Nos. 07/777,080 and 07/779,086, having thermosensitive coloring layers for magenta, cyan and yellow which are laminated or formed on a supporting material in this order from the bottom. When recording a full-color image on this type of thermosensitive color recording medium, a thermal head thermally records a yellow frame of the image in a thermosensitive recording layer which is colored in a yellow color when heated (hereinafter called simply as the yellow recording layer). The recording medium is, thereafter, exposed to light passing through a yellow filter so as to fix the yellow recording layer. Next, a cyan frame of the image is thermally recorded in a thermosensitive recording layer which is colored in a cyan color when heated (hereinafter called simply as the cyan recording layer), and then optically fixed. Finally, a magenta frame of the image is thermally recorded in a thermosensitive recording layer which is colored in a magenta color when heated (hereinafter called simply as the magenta recording layer), and then optically fixed.
Full color images often contain black-colored characters or lines. Because black is reproduced by subtractive mixture of cyan, magenta and yellow, if any of the three color separation frames deviates from a proper position, the black characters are blurred in the above-described conventional color thermal printing method.